This invention relates generally to control circuits and more particularly to a position control circuit utilizing a phase-locked loop to extend the resolution of the position control circuit.
Position control circuits are employed, for example, to control the position of an interferometrically controlled stage as described in U.S. Patent Application Ser. No. 015,713 entitled INTERFEROMETRICALLY CONTROLLED STAGE WITH PRECISELY ORTHOGONAL AXES OF MOTION, filed on Feb. 27, 1979, by Edward H. Phillips, assigned to the same assignee as the present application, incorporated herein by reference, and now abandoned in favor of continuation Application Ser. No. 198,358 filed on Oct. 20, 1980. Such position control circuits typically employ a reversible or up-down counter to provide an indication of the actual position of the stage as described in U.S. Pat. No. 3,458,259 entitled INTERFEROMETRIC SYSTEM and issued on July 28, 1969. The resolution of such position control circuits is therefore typically limited by the ambiguity of the last or least significant digit indicated by the counter.
Accordingly, it is the principal object of this invention to provide an improved position control circuit in which the ambiguity of the last or least significant digit indicated by the counter is eliminated and the resolution of the position control circuit is extended.
This and other objects, which will become apparent from a reading of this specification and an inspection of the accompanying drawings, are accomplished according to the illustrated preferred embodiment of the present invention by employing a position control circuit having a variable phase shifter responsive to an input reference signal and to a control signal for producing an output signal of the same frequency as the reference signal but shifted in phase as determined by the control signal, and by employing a phase detector responsive to the output signal from the variable phase shifter and to an input measurement signal of a frequency related to the frequency of the input reference signal for producing a position control signal extending the resolution of the position control circuit. The variable phase shifter comprises another phase detector responsive to the input reference signal and to an output signal from a divide by N circuit for driving a voltage controlled oscillator to supply the divide by N circuit with an output signal having a frequency N times greater than the frequency of the reference signal. A shift register is responsive to the output signals from both the voltage controlled oscillator and the divide by N circuit for supplying N output signals of different phase to a data selector. The data selector is responsive to the control signal for supplying a selected one of these N output signals to the first-mentioned phase detector as determined by the control signal.